A function whose declared type is neither void nor any (TS)

A function whose declared type is neither void nor any (TS) #

The "A function whose declared type is neither void nor any must return a value" error occurs when we have explicitly set the return type of a function, but the function does not return a value. To solve the error, make sure to return a value from the function or remove the return type.

Here is an example of how the error occurs.

index.ts

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// ⛔️ Error: A function whose declared type is
// neither 'void' nor 'any' must return a value.ts(2355)
const getPromise = (): Promise<number> => {
  Promise.resolve(42);
};

We've set the return type of the getPromise function to be Promise<number>, but we don't return a value from the function.

If you don't return a value from a function, the function implicitly returns undefined.

To solve the error, return a value that matches the return type of the function.

index.ts

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const getPromise = (): Promise<number> => {
  return Promise.resolve(42);
};

If your function doesn't need to return anything, you can simply remove the function's return type.

index.ts

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// 👇️ const getPromise: () => void
const getPromise = () => {
  Promise.resolve(42);
};

Now the function has a return type of void, which represents the return type of functions that don't return a value.

Sometimes you might get this error when you have a complex function with nested conditions and callbacks.

index.ts

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// ⛔️ Error: A function whose declared type is
// neither 'void' nor 'any' must return a value.ts(2355)
const getNumber = (): number => {
  function inner() {
    if (Math.random()) {
      return 100;
    }

    return 200;
  }

  inner();
};

The function above gets the same error because we've set the function's return type to be number, but the function doesn't return a value.

The inner function returns a value, but getNumber does not.

This is often a source of confusion when using callback functions - returning a value from a nested function doesn't mean that you return the value for the outer function.

The inner() function in the example returns a number, but if you want to return the number from the outer function's return value, you have to explicitly do it.

index.ts

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// ✅ OK now
const getNumber = (): number => {
  function inner() {
    if (Math.random()) {
      return 100;
    }

    return 200;
  }

  return inner();
};

Now we explicitly return the return value of the inner function.

We had to do this because you only return a value for the function in whose scope you've used the return statement (or the implicit arrow function return).

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A function whose declared type is neither void nor any (TS)

A function whose declared type is neither void nor any (TS) #

Conclusion #

Borislav Hadzhiev